1. Field
The present disclosure relates generally to low-noise amplifier (LNA) filters and, more specifically, to LNA filters for global navigation satellite system (GNSS) devices.
2. Description of Related Art
Navigation receivers that use GNSS, such as GPS or GLONASS (hereinafter collectively referred to as “GNSS”), enable a highly accurate determination of the position of the receiver. The satellite signals may comprise carrier harmonic signals that are modulated by pseudo-random binary codes and which, on the receive side, may be used to measure the delay relative to a local reference clock. These delay measurements are used to determine the pseudo-ranges between the receiver and the satellites. The pseudo-ranges are not true geometric ranges because the receiver's local clock is different from the satellite onboard clocks. If the number of satellites in sight is greater than or equal to four, then the measured pseudo-ranges can be processed to determine the user's single point location as represented by a vector, as well as to compensate for the receiver clock offset.
A more detailed description of determining a position based on signals from GNSS satellites and base stations is available in U.S. patent application Ser. No. 12/070,333, filed Feb. 15, 2008, published as US Patent Publication No. 2008/0208454 and Ser. No. 12/360,808, filed Jan. 27, 2009, published as US Patent Publication No. 2009/0189804 assigned to the assignee of the present disclosure, and each of which are incorporated herein by reference in their entirety for all purposes.
Positioning accuracy of GNSS technology is directly dependent on the accuracy of the delay measurements. Transmissions from devices operating at non-GNSS frequencies that are near GNSS frequencies can cause degraded performance of GNSS services, such as less positional accuracy, by interfering with the delay measurements.
For example, a GPS-based GNSS device may use timing information transmitted on the L1 band from 1563.42 MHz to 1587.42 MHz. Without filtering, transmissions from the next lower band from 1525 MHz to 1559 MHz could interfere with accurate measurement of the timing signals in the L1 band. Many GPS-based devices employ antenna LNA filters to remove the unwanted frequencies in bands outside of the L1 band. However, many of these GPS-based devices were designed with filters built based on assumptions about the signal strength of the transmissions in the 1525 MHz to 1559 MHz band. Specifically, as this portion of the spectrum was originally designated for satellite transmission, filters for some GPS-based devices assumed weak signal strengths for transmission in this spectrum. This assumption may no longer be accurate. For example, LightSquared is a wireless broadband company that is proposing to use the above frequency spectrum, which is just below the L1 band, to provide a nationwide high-speed wireless network. As discussed above, this spectrum was previously assigned for satellite based communication. Accordingly, many GPS-based devices were designed to only filter out inferences from this neighboring spectrum based on relatively weak signals that are transmitted from space. However, part of LightSquared's network may involve ground based transmissions that are many orders of magnitude stronger than those that originate in space.
Additionally, the frequency response of a filter is typically temperature dependent. As a result, due to changes in temperature, conventional filters may not be capable of adequately filtering signals in a neighboring spectrum. Thus, improved GNSS antenna filters are desired.